1. Field of the Invention
The present invention relates to an environment measuring apparatus which uses an element having a resistance thereof changed with a change of environment.
2. Related Background Art
FIG. 18 shows a circuit configuration of a prior art environment measurement apparatus for detecting an environment condition such as a temperature or a humidity. In FIG. 18, numeral 101 denotes a current generation circuit for supplying power (+9 V, -9 V) to respective units, numeral 102 denotes a sensor unit having a thermistor 103 as a temperature sensor and a humidity sensor 104, numerals 105, 106, and 109 denote operational amplifiers, numeral 107 denotes a rectifying detection circuit, numeral 108 denotes a filtering integration circuit, numeral 110 denotes an oscillation circuit for applying a pulse of predetermined frequency and amplitude to the humidity sensor 104 through a resistor (R11) 116, and numeral 113 denotes environment condition detection means including the operational amplifiers 106 and 109, the detection circuit 107 and the integration circuit 108. Numerals 114 and 115 denote capacitors having capacitance of C11 and C12.
In the above measurement apparatus, a floating power supply is supplied to terminals P1 and P2 of the power generation circuit 101 to generate .+-.9 V power which is supplied to the respective units in the apparatus. An oscillation output having the predetermined frequency (e.g., 1 kHz) and amplitude (e.g., 3 V pp) is supplied to the humidity sensor 104 from the oscillation circuit 110 through the resistor 116 and the capacitor 114 having the capacitance C11, and the sensor output is applied to the operational amplifier 106 having a high impedance through the capacitor 115 having the capacitance C12 and is amplified thereby, and the output of the operational amplifier 106 is rectified by the detection circuit 107 and filtered by the integration circuit 108, and the output impedance is lowered by the operation amplifier 109 and a DC output signal is produced from the terminal P4. Thus, when the resistance of the humidity sensor 104 is changed by the environment, its corresponding DC output signal is produced.
FIG. 19 shows another prior art example. Numerals 1-11, 61, 70 and 71 denote analog switches for applying a square wave signal to a humidity sensor 23, connecting the humidity sensor 23 to a reference supply 22 and connecting capacitors 12, 68, and 69 to the humidity sensor 23. Numerals 13-16 denote 2-input AND gates which are used to control a switching timing of the analog switches 6, 8, 9, and 11. Numerals 21 and 22 denote reference supplies of a fixed voltage.
Numeral 55 denotes a D/A converter for producing a comparison reference voltage of a comparator 17 used to measure a resistance of the humidity sensor 23. When a control signal is sent to a signal line 45 by control means 20, digital data is switched.
Numeral 19 denotes count means and the count thereof is used to calculate a humidity by operation means. Numerals 26, 27 and 28 denote inverter circuits which invert the inputs thereto to produce inverted outputs. The above components are connected as described below.
One end of each of the capacitors 12, 68 and 69 having the other end thereof grounded is connected to one end of the analog switch 1, 2, or 70, and the other end of the analog switch 1, 2, or 70 is connected to a signal line 31 through which a positive signal input terminal of the comparator 17, a signal input terminal of the analog switch 3 having the other end thereof grounded and one of signal input terminals of the analog switch 7 are connected. An analog signal output terminal of the D/A converter 55 is connected to a negative signal input terminal of the comparator 17 through a signal line 40.
One signal input terminal of the analog switch 8 having the other signal input terminal thereof grounded is connected to one input signal terminal of each of the analog switches 6 and 7 and one terminal of the humidity sensor 23, and one input terminal of the analog switch 11 having the other input signal terminal thereof grounded is connected to one signal input terminal of each of the analog switches 9 and 10 and the other terminal of the humidity sensor 23. The other signal input terminals of the analog switches 6 and 9 are connected to a positive terminal of a power supply 21 having a negative terminal thereof grounded through a common line 48, and the other signal input terminal of the analog switch 10 is connected to a positive terminal of a power supply 22 having a negative terminal thereof grounded through a signal line 46.
Control terminals of the analog switches 1, 2, and 70 are connected to a control signal output terminal of control means 20 through signal lines 37. Control terminals of the analog switches 7 and 10 are connected to an output terminal of the inverter 28 through a signal line 33.
Control terminals of the analog switches 6, 8, 9 and 11 are connected to output terminals of the 2-input AND gates 13-16, respectively, and one input terminal of each of the AND gates 13-16 is connected to a control signal output terminal of the control means 20 through a signal line 32. Other signal input terminals of the AND gates 13 and 14 are connected to a control signal output terminal of the control means 20 through a signal line 54. Similarly, other signal input terminals of the AND gates 15 and 16 are connected to the control signal output terminal of the control means 20 through a signal line 54'. The signal line 32 is connected to one input terminal of the 2-input AND gate 64 and an input terminal of the inverter 26. The three inverters 26, 27, and 28 form a delay inverter, and a signal output terminal of the inverter 26 is connected to a signal input terminal of the inverter 27, and a signal output terminal of the inverter 27 is connected to a signal input terminal of the inverter 28. A signal output terminal of the comparator 17 is connected to a signal input terminal of the count means 19 through a signal line 41.
The control means 20 and the count means 19 are connected by a bilateral signal line 44. The count means 19 outputs operation information to the operation means 18 through a signal line 43. The operation means 18 is connected to the control signal output terminal of the control means 20 through a signal line 42. A signal line 62 is connected to one signal input terminal of the 2-input AND gate 64, and the output terminal of the AND gate 64 is connected to a control terminal of the analog switch 3.
Referring to a flow chart of FIG. 20, an operation is explained.
During non-measurement of humidity, the control means 20 outputs square waves of duty factor of 50% of opposite phases to the signal lines 54 and 54' and outputs a signal "H" to the signal line 32 to enable the AND gates 13-16, and outputs a signal "H" to the signal line 62 to turn on the analog switch 3 to fix the potential of the signal line 31 to the GND potential. The signal line 60 is rendered to "L". When "H" is outputted to the signal line 54 and "L" is outputted to the signal line 54', the analog switches 6 and 11 are turned on and the analog switches 8 and 9 are turned off. Similarly, when "L" is outputted to the signal line 54 and "H" is outputted to the signal line 54', the analog switches 6 and 11 are turned off and the analog switches 8 and 9 are turned on. Since the signal line 33 is also "L", the analog switches 7 and 10 are turned off. Under this condition, a pulse (square wave) having a duty factor of 50% and a predetermined frequency and an amplitude which is two times as large as the voltage of the power supply 21 shown in FIG. 6 is applied to the humidity sensor 23 (S701).
A procedure for measuring the humidity is now explained.
During the non-measurement state of humidity, the control means 20 sets standard comparison reference digital data at a digital input terminal of the D/A converter 55. A signal is outputted to the signal line 37 to turn on the analog switch 1 and turn off the analog switches 2 and 7 to connect the capacitor 68 of the capacitance C2 which is optimum to measure a middle humidity range to the signal line 31 (S702).
The capacitance C2 of the capacitor 68 is set to a capacitance which fits the intended operation, for example, 6800 pF. The capacitance C1 of the capacitor 12 is set to a capacitance which fits to the measurement of a high humidity range, for example, 0.68 .mu.F, and the capacitance C3 of the capacitor 69 is set to a capacitance which fits to the measurement of a low humidity range, for example, 33 pF. The resistances of the resistors 29, 30, and 72 are set to 1 K.OMEGA., 100 K.OMEGA., and 10 M.OMEGA., respectively, with a precision of no greater than .+-.1%, to form time constants with C1, C2 and C3.
Under this condition, the control means 20 sends the "H" signal to the signal lines 32, 60 and 62 and turns on the analog switch 5. Then, it sends the "L" signal to the signal line 62 to serially connect the resistor 30 and the capacitor C2. Thus, the reference supply 22 starts to charge the capacitance C2 through the resistor 30. At this time, the control means 20 outputs the "H" signal to the signal line 44 before it outputs the "L" signal to the signal line 32 to previously reset the count means 19.
At the moment when the signal line 62 is rendered "L", the count means 19 detects the "L" level on the signal line 32 and starts the counting. (The count means 19 includes a time base). When the potential of the signal line 31 reaches the comparison reference potential corresponding to the analog output by the comparison reference digital data of the D/A converter 55, the output of the comparator 17 changes from "L" to "H", and when the count means 19 detects the inversion timing, it stops the counting and outputs the count to the signal line 43. The operation means 18 corrects the reference potential applied by the D/A converter 55 to the negative terminal of the comparator 17 relative to a theoretical value of a charge time determined by the capacitance of the charging capacitor and the resistance of the resistor used, and outputs a signal to the control means 20 to change the output data of the D/A converter 55 (S703).
Then, the signal line 62 is rendered from "L" to "H" and the signal line 32 is rendered to "L" so that all of the AND gates 13-16 are disabled producing the output "L". All of the analog switches 6, 8, 9, and 11 are turned off. The analog switch 3 is turned off and the signal line 31 floats.
At the next timing, the signal line 33 is rendered to "H" through the inverters 26-28 and the analog switches 7 and 10 are turned on so that the reference supply starts the charging of the capacitance C2 through the humidity sensor 23.
The control means 20 outputs "H" to the signal line 44 immediately before it outputs "L" to the signal line 32 to previously reset the count means 19 as described above.
At the moment when the signal line 32 is rendered to "L", the count means 19 detects the "L" level of the signal line 32 to start the counting (S704). When the potential of the signal line 31 reaches the comparison reference potential corresponding to the analog output by the comparision reference digital data of the D/A converter 55 (S705), the output of the comparator 17 changes from "L" to "H", and when the count means 19 detects the inversion timing, it stops the counting (S706) and outputs the count to the signal line 43. The operation means 18 reads the capacitance of the charging capacitor used, the comparison reference potential of the comparator 17 and the previously measured measurement environment temperature by the control means 20 through the signal line 43 and operates the data on the signal line 43 under the following condition or determines a relative humidity and an absolute temperature by the compare-inversion means by a chart. The above environment measurement is repeated by a predetermined number of times to determine average values of the relative humidities and the absolute temperatures.
When the count of the count means 19 does not reach the predetermined level, the control means 20 receives the information from the signal line 20 and temporarily returns to the humidity measurement mode and starts the measurement of humidity again after a predetermined relaxation time.
At that time, the analog switch 2 is turned on to connect the capacitance C1 to the signal line 31, and the above humidity measurement is repeated (S707, S712 and S713).
When the count of the count means 19 is above the predetermined level, the control means 20 receives the information through the signal line 44 and temporarily returns to the humidity non-measurement mode and measures the humidity again after a predetermined relaxation time.
At this time, the analog switch 70 is turned on to connect the capacitance C3 to the signal line 31 and the above humidity measurement is repeated. Before the capacitance is changed for remeasurement, the data of the D/A converter 55 which applies the reference potential to the negative input terminal of the comparator 17 is compensated in the above compensation method (S708, S714, and S715).
The resistance of the humidity sensor 23 is given by: EQU R=t/(C.times.1n(1/(1-Vref/Va)))
where
R: resistance of the humidity sensor 23 PA1 C: capacitance of the capacitors 12, 68 and 69 PA1 Vref: reference voltage of the D/A converter 55 PA1 Va: reference voltage of the reference supply 22 PA1 t: charge time to C from 0 V to Vref
The operation means 18 has a conversion map of the value of t of the resistance R and the relative humdity and determines the resistance by the comparator means (S709-S711).
FIG. 21 shows a circuit configuration of another prior art example. In the present circuit, the resistors 29, 30, and 72 and the analog switches 4, 5 and 71 connected thereto in the circuit of FIG. 19 are omitted. This circuit can similarly measure the environment humidity.
However, in the prior art environment measurement apparatuses described above, a dynamic range of the output is not attained in a low humidity range (the environment range in which the resistance of the humidity sensor is several hundreds M.OMEGA.) by the affect of a stray capacity, particularly a wiring capacity connected to the humidity sensor, and as a result, it is difficult to expand a measurable range.
Further, since a stray capacity of approximately 10 pF is inherently attached to the input terminal of the analog switch, if ten or more analog switches are connected to the capacitance under an environment condition in which the resistance of the humidity sensor reaches several G.OMEGA., the measurable range is narrowed because the capacitance increases.
In addition, in the prior art, since the performance of the humidity sensor is deteriorated when an excessive DC voltage is applied thereto, a predetermined non-measurement time to relieve the deterioration of the humidity sensor is set for each measurement and then the humidity measurement is resumed. It has been found by an experiment of the present inventors that if the previous measurement time is 100 ms or longer, for example, by the high resistance of the humidity sensor, the humidity sensor does not recover its initial performance unless the relaxation time is 10 seconds or longer. As the number of times of environment measurement increases, the humidity sensor is deteriorated more and the maintenance of the performance of the apparatus over an extended period is difficult to attain.
In an environment condition (high humidity range) in which the resistance of the humidity sensor decreases, the potential of the positive signal input terminal of the comparator slightly rises by the resistor division by the on-resistance of the analog switch connected to the charging capacitor and the resistance of the humidity sensor immediately after the start of the measurement so that the charging time of the capacitance measured by the count means is shorter than a theoretical time. As a result, the output of the apparatus represents the relative humidity which is higher than the actual relative humidity and a high precision is not attained.